Polycystic ovary syndrome (PCOS) is a heterogeneous disorder, and its definition remains fluid and controversial. PCOS is characterized by clinical and/or biochemical hyperandrogenism, and is frequently accompanied by ovulatory dysfunction and polycystic ovaries. PCOS is a diagnosis of exclusion, with other androgen excess and related disorders to be excluded. The prevalence of PCOS is 6.5-8.0% of unselected women of reproductive age, using the NIH 1990 criteria. Genetically, PCOS is a common, complex disorder. Despite repeated attempts to identify the putative gene or genes responsible for this disorder, the PCOS gene(s) remain elusive. © 2006 Elsevier Ltd. All rights reserved.